perf(chat): debounce persisted writes and re-render only the streaming message

- the Zustand persist middleware serialized every saved session and wrote to AsyncStorage on each streamed token (appendToMessage -> set); writes are now debounced (400 ms) and flushed when the app leaves the foreground
- every token re-rendered every MessageBubble, re-parsing the Markdown of the whole conversation; rows are memoized so only the changed message renders (covered by MessageList.test.tsx)

store/debouncedStorage.ts

@@ -0,0 +1,54 @@
+import AsyncStorage from '@react-native-async-storage/async-storage';
+import { AppState } from 'react-native';
+import type { StateStorage } from 'zustand/middleware';
+
+export interface DebouncedStorage extends StateStorage {
+    /** Writes every pending value right away. */
+    flush: () => Promise<void>;
+}
+
+/**
+ * Wraps a storage so bursts of writes collapse into one trailing write.
+ *
+ * The chat store is persisted on every state change and a streamed answer
+ * changes it once per token, which would serialize every saved session and hit
+ * AsyncStorage dozens of times per second.
+ */
+export function createDebouncedStorage(base: StateStorage, delayMs = 400): DebouncedStorage {
+    const pending = new Map<string, string>();
+    let timer: ReturnType<typeof setTimeout> | null = null;
+
+    const flush = async () => {
+        if (timer) {
+            clearTimeout(timer);
+            timer = null;
+        }
+        const entries = [...pending.entries()];
+        pending.clear();
+        await Promise.all(entries.map(([name, value]) => base.setItem(name, value)));
+    };
+
+    return {
+        getItem: (name) => {
+            const value = pending.get(name);
+            return value === undefined ? base.getItem(name) : Promise.resolve(value);
+        },
+        setItem: (name, value) => {
+            pending.set(name, value);
+            if (timer) clearTimeout(timer);
+            timer = setTimeout(flush, delayMs);
+        },
+        removeItem: (name) => {
+            pending.delete(name);
+            return base.removeItem(name);
+        },
+        flush,
+    };
+}
+
+export const appStorage = createDebouncedStorage(AsyncStorage);
+
+// Do not lose the last tokens when the app is sent to the background or closed.
+AppState.addEventListener('change', (state) => {
+    if (state !== 'active') void appStorage.flush();
+});

store/useAppStore.ts

@@ -1,7 +1,7 @@
-import AsyncStorage from '@react-native-async-storage/async-storage';
 import { create } from 'zustand';
 import { createJSONStorage, persist } from 'zustand/middleware';
 import { DEFAULT_SERVER_CONFIG, LMModel, ServerConfig } from '../services/types';
+import { appStorage } from './debouncedStorage';
 
 export interface ChatMessage {
     id: string;
@@ -164,7 +164,7 @@ export const useAppStore = create<AppState>()(
         }),
         {
             name: 'localaiapp-storage',
-            storage: createJSONStorage(() => AsyncStorage),
+            storage: createJSONStorage(() => appStorage),
             partialize: (state) => ({
                 theme: state.theme,
                 serverConfig: state.serverConfig,